Conventional light modules and methods for generating wavelength-converted light by means of conversion means, such as phosphors, for example, in which excitation light, e.g. monochromatic light from a laser diode, is radiated onto the phosphor and the latter emits light having a converted, usually greater, wavelength. In the case of phosphors, as is known, the problem occurs that they exhibit a decrease in the conversion efficiency with an increase in the pump power and pump power density of the excitation light. An increase in the pump power leads to an increase in the average temperature and, via the temperature dependence of the conversion efficiency, to a saturation of the output power (thermal quenching). A further cause that leads to a saturation of an output power when the pump power density is increased is intensity quenching, that is to say that a reduced population density of the lower pump level of the phosphor occurs on account of the comparatively long lifetime of the participating activator states of the phosphor. A further problem additionally consists in the fact that these effects, such as thermal quenching and intensity quenching, are manifested to different extents in the case of different phosphors. Especially red phosphors exhibit a very great reduction of the conversion efficiency when the pump power is increased.
In the case of light modules, for example for projectors, in which different phosphors are used for generating e.g. green, yellow and red light, the limitation of the pump power in the red channel leads to a lower red luminous flux that limits the total luminous flux of the projectors or adversely influences the white point. Furthermore, conventionally, the use of red phosphors with as little thermal quenching as possible is provided. However, these red phosphors then have a short dominant wavelength in the orange-red range that leads to a reduction of the size of the addressable color space. In this case, the dominant wavelength of a spectral composition should be understood to mean the wavelength at which a narrowband, monochromatic excitation in the human eye brings about the same color impression as a wider light distribution with the corresponding dominant wavelength. However, alternatively available LEDs for the red channel having a longer dominant wavelength likewise limit the total luminous flux in hybrid projectors on account of their low luminance.
In LARP applications (Laser Activated Remote Phosphor), a high optical input power is concentrated on a conversion phosphor in order to obtain converted light for use in the apparatus. For projection applications it is necessary to provide at least three primary colors—blue, green and red. Blue can easily be provided by LEDs or an exciting laser, in particular laser diodes. Green/yellow can be converted by means of Ce3+-doped garnet phosphors in accordance with the prior art (e.g. YAG:Ce or LuAG:Ce) with a good efficiency and reliability even at high activation energies. Red conversion phosphors for LARP applications suffer from various problems, however. The most customary and most tried and tested red conversion phosphors are constructed from materials based on nitridosilicates such as (Ca,Sr,Ba)2Si5N8:Eu2+ or CaAlSiN3:Eu2+. They all suffer from durability problems at high temperatures and at high optical energies, e.g. darkening or bleaching. In addition, Eu2+-activated phosphors at high optical activation energies lose conversion efficiency (decrease in efficiency).
A number of commercially available systems utilize Eu-doped nitridic conversion phosphors for the red conversion; by way of example, the Osram LARP projection module phaser P1 utilizes a (Ca,Sr)2Si5N8:Eu2+-phosphor. The phosphor has to be optimized for the specific application and operating temperatures and also for the emission wavelength provided. It has to be operated under carefully monitored limits. By way of example, it must not overheat, since the phosphor degrades otherwise. However, even then durability problems have to be taken into account. Small variations in the operating conditions can appreciably limit the lifetime of the phosphor.
In this context, WO 2014/019758 A1 discloses a device for providing electromagnetic radiation which includes a radiation arrangement for generating excitation radiation and at least one conversion element for generating conversion radiation. The conversion element includes at least one phosphor and is arranged at a distance from the radiation arrangement in a beam path of the excitation radiation. A coarse-grained nitridosilicate having a median grain size d50 in the range of 10 to 30 μm is suitable as phosphor.